All I know for sure is I had one Sulcata yearling blinded and another received eye damage we were able to prevent from being too bad.
BlindedMy feeble brain says that it was something like the UVB rays were too concentrated out of the end. If you used it horizontally they didn't cause any trouble. But it's been 6 or 7 years since I had my trouble with them, so it's hard for me to remember...Now...you watch...a buncha experts are gonna come on and correct me...
! I feel terrible for the poor guy Blinded
Great question. But not that easy to answer, because it requires background knowledge on how fluorescent lamps work and what UV is.
I barely knew any of that about UVB lightsGreat question. But not that easy to answer, because it requires background knowledge on how fluorescent lamps work and what UV is.
First, the proper name is "compact fluorescent lamp", CFL for short. CFLs aren't bad in general. There's just a lot of "once bitten, twice shy" going on. A relatively small amount of people reported that some CFLs caused eye problems or even blindness with their reptiles. What you'll be told less often though is that some non-compact fluorescent lamps (the linear tube kind) also caused the same problems. As their names imply, FLs and CFLs are actually closely related and share many of the same problems (like flicker, which is actually worse with the linear kind).
A quick primer on how (C)FLs work:
They are glass tubes filled with a low-pressure gas, mainly argon (harmless noble gas), and also small amounts of mercury (mandated to be less than 5 mg). Mercury is a crazy metal that is liquid at room temperature, and very toxic. That's why FLs don't belong in the general waste but have to be disposed of properly, and if they break you should follow proper clean-up procedures. No need to panic, though, if that happens. Nowadays, many CFLs use amalgam, an alloy of mercury and other metals, which is solid and therefore not as volatile as liquid mercury - at least when the lamp is "cold" and didn't break while operating.
Anyway, once an FL is activated, electricity is used to heat and vaporize the mercury (whether liquid or amalgam) inside the glass. The mercury atoms / ions get excited and release energy in the form of light. Most of that light however is in the "ultraviolet C" (UVC) part of the spectrum, which is harmful and cannot be seen by reptiles and humans anyway. Therefore, the inner walls of the glass tubes are coated with certain chemicals, called phosphor. The phosphor acts as a sort of light converter, it absorbs the UVC, and in turn releases light of longer, less harmful wavelengths. That process is called "fluorescence" and the origin of the lamps' name, of course. Seems like quite a roundabout way of producing light, doesn't it?
Now a bit more on UV:
Ultraviolet is the part of the electromagnetic spectrum adjacent to the part of light that it is visible to us. We are interested in it, because a certain amount of UV is required by skin cells in in order to synthesize vitamin D3, which plays a role in the absorption of calcium and some other elements (think bone growth).
The wavelengths we can see range from about 700 nanometers (nm), which would be a deep, dark red, to about 400 nm, a deep, dark violet. Ultraviolet literally means "beyond violet" and is used for the range from 400 nm to 10 nm. It is commonly subdivided into A, B, and C parts.
- UVA, 400-315 nm: passes almost unhindered through Earth's atmosphere, and through common glass, is visible to reptiles (down to 350-320 nm, depending on the species). Human skin overexposed to UVA ages prematurely and gets wrinkly faster.
- UVB, 315-280 nm: is mostly absorbed by Earth's ozone layer, is blocked to a large extent by common glass, is invisible. Vitamin D3 synthesis needs wavelengths below 315 nm, peaks around 300 nm. However, wavelengths at 300 nm and shorter also start damaging DNA. See this chart. Human skin overexposed to UVB gets sunburn.
- UVC, 280-100 nm: is completely absorbed by Earth's ozone layer and atmosphere, blocked by common glass, doesn't even get very far in air (a couple of inches, maybe a foot). The light emitted by mercury is mainly in this range, at 185 nm and 254 nm.
With all this in mind, here are three reasons why CFLs could be dangerous:
- Exactly what kind of light a (C)FL emits depends on the formulation of its phosphor. Tests done by this website revealed that some lamps used a phosphor type called "FS" or "UVB313", which produced the bulk of the output around 313 nm. That's UVB and it will strongly stimulate vitamin D production, but its peak is invisible to reptiles and entails a sizeable output of dangerous UVB below 300 nm, even ranging into UVC territory. For comparison: output of potentially harmful UVB313 phosphor versus output of UVA340 phosphor.
Note that this does not just apply to CFLs, there were also some "bad" linear FL tubes with that phosphor!- To achieve their compactness, CFLs have thinner glass tubes in more complicated shapes, particularly the helical "coil" ones. There was a study by Rafailovich et al. which reported "cracks" in bendy bits of the phosphor coating, meaning that CFLs with such damaged coatings can leak UVC from the original mercury emission. That makes for very media effective headlines about "skin-frying" CFLs, but the methods of that study aren't undisputed, and if you remember, UVC is absorbed quite strongly by common glass and even air. Therefore, unless you're closer than one or two foot to a CFL, that's unlikely to cause problems. Incandescent lamps and even fires emit a bit of UV as well at such close ranges. Reptiles in their enclosures on the other hand are often much closer to lamps than we are, so this is a more pressing concern for them. Make sure that there's always a sufficient safety distance between lamp and animal. It might be better to err on the side of "too far".
- Orientation and reflectors. Orientation was mentioned here before, but unlike Maggie said, the "end" of a CFL does not have a concentrated output - at least not without a reflector. Imagine looking straight at the tip of a CFL versus looking at its side. The tip has a much smaller area, and therefore emits less light in total. This diagram here shows clearly how a vertically oriented CFL emits an almost donut-shape of light, more to the sides than downward. If you oriented it horizontally, you'd increase the intensity at any given distance below, not decrease it. And you'd "waste" practically half of the output, because it'd go upward, out of the enclosure. That's where reflectors come in. They are meant to redirect the lamp's output downward to where it's needed. However, they can actually be too good at that! The more mirror-like the inside of a dome-shaped reflector, the stronger its effect. By using the wrong reflector, you can potentially turn a harmless lamp into a UV death ray. So, better don't experiment with that, unless you have a UV meter.
In short: don't buy lamps with "bad" phosphor (difficult to determine, maybe from the spectral output chart on the packaging, if that's present and trustworthy). Don't position the lamp too closely to the animal (again, difficult to determine the optimum distance). Don't use a reflector dome, at least not a highly reflective one.
Or just don't use any UV lamps at all, instead build your tortoises a nice outdoor enclosure in a sunny spot.
! That sums it up pretty well. Why do they still make these things??? I never knew lights could be so dangerous 
We have a pretty good idea why - excessive amounts of UV radiation, mainly in the invisible UVB region.
Why cant it be in the UVA region? An excess there can be just as bad...
Then don't exaggerate.
Simple: because the animals can see UVA (well, at least partially). If a lamp with mainly UVA output is too close / too intense, the animals should perceive that and try to hide in the hopefully present shaded areas. If a lamp with mainly UVB output is too close / too intense, they won't be able to tell from the less intense UVA that such a lamp outputs. Just like our ears can get damaged by strong ultrasound even though we can't hear it because its frequencies are too high.
They do go into hiding, a usual complaint is the tortoise is always hiding. The tortoise may sense that the UVA is intense, but it might not cause an immediate threat to the tortoise (or at least thats how they may perceive it). Isn't this possible too?
Can you check if any if the harmful ones contain any UVC light? Or is that impossible? I'm just curious...
I can't. No. That is why I'm trying to collect the bad bulbs and get them sent to a lab with the right equipment and a person with the skill set needed to properly test them.
